I've been working on the current-limiting part of a linear lithium-ion charger, which is very similar to the eevblog constant current load, for which there are countless threads on this forum already. So I read through many of them and came up with the attached circuit.
I quickly wired it up on a breadboard, without the noise-limiting components (R5, R6, C1) or the protection diodes (so as to keep it a simple test), and it works as expected. I used an LM7812 for voltage regulation because the mosfet I'm using isn't logic-level and I'm going to be charging 4S packs and above anyway. The D2 diode is in there in case the charging source is a battery itself. I used the LM358 because is the op-amp I'm most familiar with (which is to say, just a little more than completely unfamiliar with).
I have the following questions about the circuit:
1) Is there a reason not to use a very low-value sense resistor (0.1ohm or lower) to keep the heat loss through it to a minimum?
2) Since the load is a "component" which will change its voltage, the heat output of the mosfet will be greatest when the charging begins, and decrease as it progresses. If I'm charging a 5S lithium-ion pack from ~15V to 21V, the mosfet will start the charging process dissipating Vin-15*A watts of heat, and by the time it gets to the constant-voltage stage it will be dissipating Vin-21*A watts of heat. Is this correct?
3) The voltage drops from input to output so far are: ~1V for 200mA, ~1.8V for 500mA, and ~3V for 1A. Is this more to do with the characteristics of the circuit, the characteristics of the mosfet, the relatively high value of the sense resistor, or just the fact that it's on a breadboard and the connections are crappy?
Any other suggestions are welcome. Thanks.